//使用位操作显示二进制
#include<stdio.h>
#include<limits.h>
char * itobs(int, char *);
void show_bstr(const char *);
int invert_end(int num, int bits);

int main(void)
{
    char bin_str[CHAR_BIT * sizeof(int) + 1];
    int number;
    
    puts("Enter integers and see them in binary.");
    puts("Non-number input terminates program.");
    while(scanf("%d", &number) == 1)
    {
        itobs(number, bin_str);
        printf("%d is\n", number);
        show_bstr(bin_str);
        putchar('\n');
        number = invert_end(number, 4);
        printf("Inverting the last 4 bits gives\n");
        show_bstr(itobs(number, bin_str));
        putchar('\n');
    }
    puts("Bye.");
    return 0;
}

char * itobs(int n, char * ps)
{
    int i;
    const static int size = CHAR_BIT * sizeof(int);
    for(i=size-1; i>=0; i--, n>>=1)
        ps[i] = (01 & n) + '0';
    ps[size] = '\0';

    return ps;
}

void show_bstr(const char * str)  //以4位为一组，显示二进制字符
{
    int i = 0;
    while(str[i])
    {
        putchar(str[i]);
        if(++i % 4 == 0 && str[i])
            putchar(' ');
    }
}

int invert_end(int num, int bits)
{
    int mask = 0;
    int bitval = 1;
    while(bits-- > 0)
    {
        mask |= bitval;
        bitval <<= 1;
    }

    return num ^ mask;  //按位异或
}